Ben E Gas, my late cat, BeeBee, was much like your kitten. She was certainly never abused but being anti-social around anyone but me was just part of her personality. She, too, would hiss at other people and be generally cranky. A friend of mine always said that BeeBee would have been happy if everyone in the world (except me) would just die. No one ever saw just how loving and fun BeeBee was when it was just the two of us - they just had to take my word for it. Don't worry that your cat doesn't warm up to everyone else - take it as proof that you are special to him and that he loves you.